This invention relates generally to jet propulsion engines, and more specifically to compressor stator vane airfoils used therein.
In a gas turbine engine air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. The combustion gases are discharged through turbine stages which extract energy therefrom for powering the compressor, and producing output power for use in driving a booster (low pressure compressor) and a fan in an exemplary turbofan aircraft engine application.
A multistage axial compressor includes cooperating rows of stator vanes and rotor blades which decrease in size to pressurize air in stages. The compressor vanes and blades have corresponding airfoils which typically vary in configuration as their size decreases from stage to stage for maximizing performance of the compressor. Compressor performance includes, for example, efficiency of compression, flow capability, and stall margin, which are all affected by the aerodynamic configuration of the airfoils of the vanes and blades, as well as by the outer and inner flowpath boundaries on the stator vanes.
More specifically, the flow or pressure distribution of the air as it is being compressed through the stator vanes and rotor blades is a complex three dimensional flow field varying circumferentially around the compressor, radially along the span of the vane and blade airfoils, and axially along the circumferentially opposite pressure and suction sides of the airfoils. Actual flow of the air being compressed through the stages is not uniform over the radial span of the blades and vanes since the outer and inner flowpaths provide substantial interaction with the axially moving airflow. The stator vanes must efficiently diffuse the flow and deliver it with proper velocities to the downstream rotors. Flow in the end wall region is complex where boundary layers on the vane and flowpath surfaces come together. Good stator designs must be compatible with the requirements of the rotors, without producing flow separations that would limit the operating range of the compressor.
Axial and mixed flow compressor blades that are designed to compress the air usually have a rotor or number of rotors that rotate inside a stationary casing and act to raise the total pressure and temperature of the flow passing through the machine. The compressor rotor blades carry a lift on the body of the airfoil that manifests itself as a higher static pressure on the pressure surface of the airfoil and a lower static pressure on the suction surface of the airfoil. Generally a small gap exists between the tip of the compressor rotor and the radially adjacent casing flowpath. The pressure difference between pressure side and suction side of the airfoil drives flow through the tip gap of the compressor rotor. This tip flow can roll up into a vortex, which tends to collect on the pressure side surface of the circumferentially adjacent blade, leading to high levels of loss and blockage in the compressor tip region. As this blockage spreads across the compressor rotor tip, the ability of the compressor to produce a pressure rise decreases, and may result in a stall in some cases. This problem is exacerbated by weak flow near the endwalls (such as the outer and inner flowpath boundaries on the stator vanes) caused by cumulative effects of skin friction and secondary flows. Weak flow near the endwalls allows the aforementioned vortices to remain in the rotor tip region, rather than being convected downstream.
In the art, in conventional designs, it is generally accepted that pressure and velocity profiles will be somewhat weak in the endwall regions (such as the outer and inner flowpath boundaries on the stator vanes) due to the aforementioned loss mechanisms. Stator vanes are generally bowed in the span-wise direction, with moderate increases in camber near the end walls that conventionally do not exceed 1.4 times the minimum value.
Accordingly, it would be desirable to have a compressor stator vane having specific features that improve stator operation with rotors that are designed to strengthen the flow in the end wall regions and can reduce weak flow near endwalls in the compressor. It is desirable to have a compressor stator vane having an airfoil that can strengthen endwall flow speed and pressure to facilitate reduction of tip blockage in the rotor and thereby increase throttle margin.